The present invention relates to control systems of the type having a plurality of remotely located process control units connected together through a communications link and, more particularly, to a control system in which each of the remote units sequentially assumes supervisory communication control of the communication link and in which high reliability information transfer is achieved between remotes.
Many system type industrial installations, for example, those related to industrial process-type manufacturing and electrical power generation, employ a large number of physically distributed controlled-devices and associated sensors for effecting coordinated operation of the overall system. In the past, coordinated control of the various devices has been achieved by manual operation and various types of semi-automatic and automatic control systems including electromagnetic relay systems, hardwired solid-state logic systems, and various types of computer control systems. The computer systems have included central systems in which the various sensors and controlled devices are connected to a central computer; distributed control systems in which a remotely located computer is connected to each of the controlled devices and to one another; and hybrid combinations of the central and distributed systems. The successful functioning of the control system is vital to any industrial process, and, accordingly, distributed systems have generally been preferred over central systems because the failure of one of the remotely located control computers generally does not cause a system wide failure as in the case of the failure of the central computer in the central system. However, in many distributed computer systems, one of the remotes or a specially designed control unit generally handles supervisory communication control of the communication buss and, for these systems, failure of the communication buss supervisor can lead to a system-wide failure.
In many industrial control systems, the various communication busses that extend between the remotely located computer process control units are exposed to high electrical noise environments. Accordingly, the information transferred over the communication buss can be subjected to error-inducing interference because of the harsh electrical environment. In view of this, a control system must have a means for detecting errors within the transmitted information in order to provide high reliability data transmission between remotes.